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Creature Feature: The Desert Tarantula Looks Big, Hairy, and Scary
The desert tarantula is one of the most misunderstood creatures in the arid Southwest. Despite its fearsome appearance, this fascinating arachnid is docile, reclusive, and nearly harmless.
The desert tarantula (Aphonopelma chalcodes) -- aka western desert tarantula, Arizona blond tarantula or Mexican blond tarantula -- is one of about 50 species of terrestrial tarantulas native to the southwestern and central United States.
The name "tarantula" was originally applied to a European wolf spider (Lycosa tarantula) that lived near the southern Italian town of Taranto. The name was borrowed and applied to some of the large spiders that Europeans encountered in the New World.
This big spider's primary range includes arid portions of Arizona, New Mexico, Southern California, and adjacent parts of Mexico. Within this limited range it is actually very common, especially in open shrublands and sparse grasslands with dry, sandy, well-drained soils. Creosote bush scrub and Joshua tree forest are two examples of ecosystems that provide excellent habitat for desert tarantulas.
Tarantulas inhabit many national parks, including (to name just a few): Saguaro National Park, Organ Pipe Cactus National Monument, Joshua Tree National Park, Death Valley National Park, Zion National Park, and Grand Canyon National Park.
You'll have no trouble recognizing tarantulas when you see them. They've been succinctly described as "big, hairy, and scary." Male and female tarantulas are about the same size and color (varying by subspecies; typically dark brown to nearly black), and both genders are conspicuously large. A mature tarantula weighs several ounces and has eight four-inch long legs appended to a body that's two or three inches long.
Some awed observers have been moved to exclaim that a tarantula looks "as big as a dinner plate." Well, a saucer, anyway.
Like all spiders, desert tarantulas are invertebrates that rely on an exoskeleton for muscular support. This means that tarantulas must shed their rigid exoskeletons periodically in order to grow. Young tarantulas molt several times a year, but mature ones usually molt only about once every year or whenever they have to replace lost limbs or urticating hairs.
What makes tarantulas look hairy is the thousands of hair-like filaments or fine bristles that cover much of their bodies. The main function of these hairs (called setae) is to help the spider, which has very poor eyesight, sense the presence of prey, determine wind direction, identify chemical signatures (such as pheremones), and assess other vital elements of its environment.
Hundreds of the hairs on the upper abdomen also have defensive functions. Called urticating hairs, they are tipped with backward-pointing barbs that are very irritating and difficult to remove if embedded in the eyes, face, paw, or other body part of any creature that molests it. They are even more damaging if deeply inhaled. Being fragile, urticating hairs break off at the slightest touch. A tarantula that is cornered or molested will raise its front legs and rapidly move its hind legs to flick urticating hairs onto any animal that's bothering it.
Tarantulas also have special "taste hairs" around their mouth and near their feet.
As if the tarantula's big and hairy appearance weren't enough to push arachnophobia sufferers over the edge, this spider also has two large, conspicuous fangs for injecting venom into prey. The venom not only helps to immobilize prey, but also liquefies and digests the soft insides of the beetles, crickets, grasshoppers, cicadas, caterpillars, millipedes, spiders, and other animals in the tarantula diet (which may include lizards, snakes, mice, and small birds if the spider is big enough). Since a tarantula can't eat solids, it's diet basically consists of "bug soup."
Many people think that tarantulas are quick to bite people, and that the bites are very dangerous. Neither assumption is correct. Since a tarantula has a rather gentle disposition, and would rather flee than fight, it rarely uses its fangs for anything except catching and mashing up prey. And while it is true that a tarantula may inflict a painful bite on a human if sufficiently provoked, the bite hurts less than a bee sting and poses no serious hazard unless the bite becomes infected or the victim has a severe allergic reaction.
Paradoxical as it may seem, the tarantula's hairs may very well pose a greater hazard than its venom. It isn't just that imbedded hairs are very irritating and difficult to remove from your skin. Hairs that get into your eyes can cause ocular injuries that persist for some time, and hairs that you inhale into your breathing passages can cause allergic rhinitis with symptoms such as a runny nose, congestion, sneezing and sinus headaches. Hairs that you deeply inhale may cause even more serious lung problems.
The fact that tarantulas pose little appreciable threat to humans hasn't prevented us from demonizing them in popular culture. Like wolves and sharks, tarantulas have been cast in the role of ferocious, thoroughly repulsive predators. Hollywood films are a good case in point. Tarantulas (1955) tells of an experiment-gone-wrong that turns a tarantula into a giant monster that must be dispatched with napalm. In The Incredible Shrinking Man (1957), a man accidentally shrunk to toy size must fight and kill a menacing tarantula with a common straight pin. In the 1962 spy classic Dr. No, James Bond (Sean Connery) uses a shoe to dispatch a tarantula that the villains snuck into his bed to kill him. Kiss of the Tarantula (1976) is a horror movie that has the teenage daughter of a mortuary operator unleashing pet tarantulas against her foes. More recently (1990), Arachnophobia had killer tarantulas terrorizing a California town.
The Goliath Birdeater (Theraphosa blondi), a tarantula that lives in Venezuela and Brazil, is the world's largest spider. It reportedly can weigh as much as 5.3 ounces and has a leg-span of up to 12 inches.
In the real world, tarantulas don't stalk you like they do in the horror films. You have to find these solitary, reclusive animals, and since they are nocturnal the best time to do that is during the cooler and darker hours. After spending the day hiding in their silk-lined burrows (or perhaps under a rock), where they are tolerably cool and safe from predators, tarantulas come out to hunt as sunset approaches and return to their burrows near sunrise. Although the big spiders are adept at ambushing passing prey by popping out of their burrows, they are also skillful stalkers.
Desert tarantulas seen out and about in the daytime, perhaps while crossing roads within a few hours before sunset or after dawn, are likely to be mature males about ten years of age that are on the prowl for females. The breeding season varies from June to December in different locations, but commonly peaks during autumn. Females remain hidden in their burrows during the day, letting the males come to them.
Sexing tarantulas isn't too difficult if you know what to look for and where. Males have a tibial spur on each front leg behind the "knee." Females lack this claw-like appendage.
After mating, the female deposits up to 1,000 eggs in a silken egg sac and aggressively guards them during a gestation period of six to seven weeks. After hatching, the spiderlings (commonly numbering about 600) remain in the nest only a few days before dispersing. Most die long before they get the chance to mature, but a few survive to perpetuate the species.
Although mature desert tarantulas have few natural predators, birds do kill a significant number. A parasitic wasp called the tarantula hawk also poses a significant hazard to the big spiders. The big wasps (which may be as large as a hummingbird) inflict a very gruesome death, too. A spider unfortunate enough to be chosen as a paralyzed host for this insect's single egg gets eaten alive, from the inside out, after the egg hatches into a voraciously hungry grub.
Having remained reasonably abundant throughout their range, tarantulas are by no means endangered. Threats posed by human activities are minimal. There is some habitat loss due to development, of course, and some tarantulas (usually breeding age males) are killed crossing roads.
Postscript: Tarantulas have become popular as exotic pets because they are gentle and easy to maintain. Females are most desirable because they live longer, up to 20 years or so. The males caught in the wild are likely to be about ten years old (breeding age) and can't be expected to live much more than a year after capture.